1. Field of the Invention
A method for dynamically measuring the instantaneous stretch of a seismic streamer cable section as it is towed through the water behind a survey ship.
2. Discussion of Related Art
During the course of conducting a marine seismic survey, a survey ship such as 10, FIG. 1, tows a long instrumented streamer cable 14 through at or just below the surface of a body of water 12. Typically, the cable 14, which may be several kilometers long, is comprised of many separate active instrumented sections, such as 18, 18', 18", 18'" each about 75-100 meters long. The cable includes a plurality, such as 3000, of hydrophones (not shown) distributed inside the cable at preselected spaced-apart intervals therealong. The hydrophones may be divided into a sub-plurality of electrically-interconnected groups of several hydrophones each. The hydrophone groups convert compressional seismic wavefields to electrical signals which are transmitted via electrical conductors, not shown in FIG. 1, that extend along the length of the streamer cable, to archival storage electronics 20 in ship 10 following partial processing in electronics modules that are incorporated in the cable sections. The hydrophones and the electrical conductors are enclosed in a jacket, 22 (FIG. 2) made of a selected plastic such as polyurethane which may be filled with a buoyancy medium. Inside the jacket, the streamer cable includes one or more longitudinal stress members of steel or aramid fiber. Although steel stress members exhibit virtually no stretch, aramid fibers are known to have a modest modulus of elasticity.
At sea, wind and waves cause the towing ship to lurch and roll. To prevent application of undesirable jerk forces to the seismic cable, which introduce catastrophic noise to the seismic data signals, an inert isolator section 16 is inserted between the ship 10 and the streamer sections 18.sup.1 to mechanically decouple the towing ship from the streamer cable itself. The resilient isolator section, to be discussed in detail later, consists of a plastic jacket, as with the active sections. It contains the electrical conductors and floatation means but usually no hydrophones. The isolator section may be 50-250 meters long and is capable of stretching nearly twice its own length. Elastic stress members, such as polyamide-fiber rope form part of the isolator section to limit the total stretch within preselected limits.
Three-dimensional seismic surveys require accurate knowledge of the exact spatial location of the hydrophones so that subsurface earth structure can be precisely modeled. Customarily, a precision navigation system defines the exact ship's coordinates. The hydrophone locations are then referenced with respect to the ship. The isolator section is beset with constantly-changing towing forces. Because of those forces, the length of the isolator section continuously varies by as much as 150%. Therefore, the instantaneous relative coordinates of the hydrophones in the streamer, being unpredictable with respect to the ship's position, must be monitored continuously.
One method for monitoring the disposition of one or more streamer cable sections and its accompanying hydrophones relative to the ship makes use of acoustic pingers wherein a master acoustic transducer on the ship interrogates slave transducers mounted in the streamer cables. The streamer cable configuration is determined by acoustic ranging based on the travel time of an interrogation pulse propagating through the water between the master transducer and the respective slave transducers. See for example U.S. Pat. Nos. 4,726,315 and 4,781,140, issued Feb. 23, 1988 and Nov. 1, 1988 respectively, to Robert R. Bell et al. and assigned to Teledyne Exploration Inc. Although acoustic ranging provides a first approximation of the ship-hydrophone distance, the method is fraught with error due to an imperfect knowledge of the velocity of sound through the water. That quantity varies continuously as a function of salinity, temperature and other water properties.
Linear inductive-type transducers are known but the useful stroke length is too short to be of value for this application. One such device is disclosed in U.S. Pat. No. 4,386,386, issued May 31, 1983 to Sigeyuki Akita. This device is a capacitor type displacement or load sensor that acts a variable-gap capacitor, the capacitance of which is a function of displacement.
There is a need for an accurate means for measuring the instantaneous length of an isolator section as well as variations in the length of the streamer sections themselves, particularly when aramid fibers are used as stress members in the active sections.